Variability of atmospheric krypton-85 activity concentrations observed close to the ITCZ in the southern hemisphere

Krypton-85 activity concentrations in surface air have been measured at Darwin, which is located in northern Australia and is influenced by seasonal monsoonal activity. Measurements between August 2007 and May 2010 covered three wet seasons. The mean activity concentration of krypton-85 measured during this period was 1.31 ± 0.02 Bq m−3. A linear model fitted to the average monthly data, using month and monsoon as predictors, shows that krypton-85 activity concentration measured during the sampling period has declined by 0.01 Bq m−3 per year. Although there is no statistically significant difference in mean activity concentration of krypton-85 between wet and dry season, the model implies that activity concentration is higher by about 0.015 Bq m−3 during months influenced by the monsoon when a north westerly flow prevails. Backward dispersion runs using the Lagrangian particle dispersion model Hysplit4 highlight possible source regions during an active monsoon located deep in the northern hemisphere, and include reprocessing facilities in Japan and India. However, the contribution of these facilities to krypton-85 activity concentrations in Darwin would be less than 0.003 Bq m−3.